1. Field of the Invention
The present invention relates to a method and an apparatus for finishing the surface of a gear tooth which has a significantly reduced level of noise and heat generation during its operation.
2. Description of the Prior Art
Machine tools and automobiles incorporate large numbers of gears. These gears are formed by a gear cutting process using a gear hobbing machine or a gear shaper and by a subsequent tooth surface finishing process with a gear grinding machine which uses a gear shaving tool or a grinder.
In such a known tooth surface finishing method or apparatus, the accuracy of the finished shape of tooth surface largely depends on the accuracy of the shape of the shaving tool or grinder, the quality of the tools of the various machines used and the accuracy by which their rotations are synchronized. The finishing of a tooth surface performed by a gear grinding machine suffers from problems such as irregularities, or tool marks, formed at intervals on the tooth surface corresponding to a fixed feed pitch, and the shapes of the parts provided near the tooth are limited by the necessity of providing a relief for the grinder.
FIG. 1 is side view of a part of a workpiece gear 1.
FIG. 2 is a cross-sectional view taken along the line II--II in FIG. 1. In finishing the tooth surfaces 30 of the gear 1, a finishing tool, for example, a grinding tool, is moved in a tooth trace direction, a direction substantially perpendicular with respect to the drawing sheet in FIG. 1. In other words, the tooth trace direction shown by the arrow 6 in FIG. 2 is used for grinding to finish the tooth surfaces 30.
According to such a finishing method, unevenness of the surface of the tooth surface 30 caused by a pitch error and a tooth profile error can be corrected with accuracy in the order of microns. However, since the grinding tool is moved in the tooth trace direction 6 in FIG. 2 in this finishing method, lateral unevenness of the surface of the grinding tool still remains on the tooth surface 30 as shown by e,v in FIG. 3. Although this surface unevenness remaining on the tooth surface 30 can be decreased by leveling the surface of the grinding tool, the grain gaps on the surface of the grinding tool will be clogged shortly into a state such that the shape of the gear 1 cannot be accurately corrected. Therefore, it is difficult to finish each of the tooth surfaces 30 of the gear 1 into a configuration of a smooth continuous single curve or line.
Further, since the grinding tool has generally slight dropouts itself and since the unevenness generated on one of the tooth surfaces 30 of the gear 1 differs from that generated on the other, it is extremely difficult to finish each individual tooth surface 30 exactly identical. Therefore, it is the present state of the art that the tooth surface 30 of each tooth of gear 1 are finished into different shapes. In other word, the tooth surfaces 30 are not satisfactory with respect to their relative surface roughness along the tooth surface curves, for example, involute curves, and have great non-uniformity in their final shapes.
FIG. 4 shows another prior art method of fishing a workpiece gear 1 by engagement with a master gear 2. In this method, it is also known to rock or vibrate the master gear 2 along a line X intersecting both center axes of the workpiece and master gears 1, 2. By this method, it is possible to slightly improve the finishing grade of the gear surface 30.
However, there are still difficulties with the above method, for example, the actual contact point C of the meshing gear surfaces of the gears 1 and 2 provides a contact with a specific angle .alpha.. This angle .alpha. is called a pressure angle. Therefore, if gears 1 and 2 are vibrated along the center axis X, gear surface 30' of master gear 2 knocks into, does not slide over, the gear surface 30 of the workpiece gear 1 so that the resulting rocking motion causes an unnecessary unevenness on the gear surface 30. As a result, the gear surface 30 is not finished with an ideal involute curve.